This application claims the priority of German Patent Document No. 101 35 003.1, filed Jul. 18, 2001, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a compressor casing structure in the region of a rotor blade ring through which there is an axial flow, having a multiplicity of grooves which are distributed uniformly over the circumference of the casing, are open towards the blade tips and extend at least approximately axially.
A compressor casing structure of this type is known, for example, from German Patent Document No. DE 35 21 798 C2 and primarily has the function of raising the pumping limit when there is increasing throttling in the partial load mode or full load mode in order to permit reliable operation without pumping or in order to permit the available operating range to be increased. The grooves act here as recirculation channels for built-up air under high pressure, which would lead to rotating stall and pumping in the outer region of the rotor blade ring without a recirculation facility. Here, the upstream, front groove ends are located upstream of the blade-tip inlet edges (see dimension A in FIGS. 2, 8, 9 and 10), the rear groove ends lie in the radial plane of the blade-tip outlet edges or just before this radial plane. According to FIG. 4 of this patent document there is provision for the grooves which are straight per se to be arranged inclined in the circumferential direction in such a way that the ingress of air is facilitated at their downstream ends (see also claim 2 in this respect).
A further measure in order to improve the ingress of air is to position the grooves/slits obliquely at an angle with respect to the compressor longitudinal center axis (see FIG. 3 and claim 3).
European Patent Document No. EP 0 497 574 B1 protects a compressor casing structure (fan case treatment), which is arranged over the blade tips of a low-pressure compressor. This structure comprises inlet and outlet passages (34, 36) or inlet and outlet openings (56, 58) which are spaced apart axially and vanes (38, 66) in the connecting passages between the inlet and outlet. The recirculation air which enters the structure with a significant circumferential component is deflected by the vanes in such a way that it is fed back into the main stream through the outlet in a predominantly axial direction, i.e., largely without a circumferential component. Without this change or reduction in the circumferential component, the air would strike the rotor blade tips with a swirl opposed to the rotation of the blade tips, i.e., with a significant angular deviation from the blade entrance angle at the pressure side, associated with flow losses and an increased tendency towards hydraulic stalling on the suction side. This disadvantage, which still occurs in certain embodiments of DE 35 21 798 C2, is avoided according to EP 0 497 574 B1. However, the structural complexity with separate inlet and outlet openings as well as a multiplicity of vanes is very high and can certainly only be implemented with geometrically large compressor blades and casings.
In view of the above, the object of the present invention is to make available a compressor casing structure which is based on the principle of the circulation of air and gas and which permits the pumping limit of a compressor to be raised significantly, thus making possible a perceptible increase in its working range through hydraulic optimization, with a simple, cost-effective design.
The present invention uses grooves which are open towards the rotor blade tips and whose openings extend at least approximately axially in the outer annular space contour. In contrast to known solutions, the groove cross sections are however continuously swirled from the upstream groove ends as far as the downstream groove ends, i.e., their angle of inclination with the radial component and circumferential component changes uninterruptedly over the length of the groove, there being a point with a purely radial cross sectional orientation approximately in the axial center of the groove, that is to say a xe2x80x9czero cross-overxe2x80x9d of the angle of inclination. The groove cross sections are inclined at the downstream groove ends in such a way that the entry of the recirculation air is made easier, the inclination from the opening to the groove base having a circumferential component in the direction of rotation, i.e., in the direction of movement of the blade tips. At the upstream groove ends, the inclination is reversed so that the recirculation air which emerges here into the main stream strikes the rotating rotor blade tips in a co-rotating fashion, which significantly improves the application of the flow and reduces losses. The tendency towards breaking away of the flow is also markedly reduced.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.